1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a reconstitutable fuel assembly skeleton which after being repaired has both structural and non-structural top nozzle/guide thimble joints.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper insert or sleeve for attachment to the top nozzle.
During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. In response to the high costs associated with replacing fuel assemblies containing defective fuel rods, recently, reconstitutable fuel assemblies have been developed. The general approach to making a fuel assembly reconstitutable is to provide it with a removable top nozzle.
One approach to top nozzle removal is illustrated and disclosed in European Patent Application No. 0 036 821. In one embodiment of this approach, as shown in FIGS. 3, 6 and 7 of the application, the guide thimbles are severed just below the top nozzle adapter plate to allow removal of the top nozzle. Then, a new top nozzle equipped with short tubes having diameters slightly larger than the outside diameter of the guide thimbles are applied to the upper severed ends of the guide thimbles and bulged fitted thereto. However, this approach disadvantageously requires the use of a new top nozzle and only allows the fuel assembly to be reconstituted once.
Another approach to reconstitutable fuel assembly construction is illustrated and described in the first application cross-referenced above. It incorporates a releasable attaching structure for removably mounting the top nozzle on the upper ends of the control rod guide thimbles. Various tools and fixtures used in releasing and installing the attaching structure for removing and replacing the top nozzle are illustrated and described in the second through fifth cross-referenced applications. A recently devised, reusable locking tube for the attaching structure is illustrated and described in the sixth application cross-referenced above.
The attaching structure includes a plurality of outer sockets defined in an adapter plate of the top nozzle, a plurality of inner sockets with each formed on the upper end of one of the guide thimbles, and a plurality of removable locking tubes inserted in the inner sockets to maintain them in locking engagement with the outer sockets. Each outer socket is in the form of a passageway through the adapter plate which has an annular groove. Each inner socket is in the form of a hollow upper end portion of the guide thimble having an annular bulge which seats in the annular groove when the guide thimble end portion is inserted in the adapter plate passageway. A plurality of elongated axial slots are provided in the guide thimble upper end portion to permit inward elastic collapse of the slotted portion so as to allow the larger bulge diameter to be inserted within and removed from the annular circumferential groove in the passageway of the adapter plate. In such manner, the inner socket of the guide thimble is inserted into and withdrawn from locking engagement with the outer socket.
The locking tube is inserted from above the top nozzle into a locking position in the hollow upper end portion of the guide thimble forming the inner socket. When inserted in its locking position, the locking tube retains the bulge of the inner socket in its expanded locking engagement with the annular groove and prevents the inner socket from being moved to a compressed releasing position in which it could be withdrawn from the outer socket. In such manner, the locking tubes maintain the inner sockets in locking engagement with the outer sockets, and thereby the attachment of the top nozzle on the upper ends of the guide thimbles.
Although unlikely, the possibility exists that during a fuel assembly reconstitution operation, while the top nozzle is removed from the irradiated fuel assembly to provide access to any defective fuel rod, the upper end portion, or insert sleeve, of the guide thimble could be inadvertently damaged by tooling or by a dropped object which strikes the guide thimble upper portion. Should such a mishap occur, the fuel assembly could be rendered unacceptable for further reactor service if the damaged portion cannot enter the respective top nozzle adapter plate passageway and be properly secured by the respective locking tube.
Consequently, there is a need for a workable repair approach to rectify (i.e., repair or remove) and accommodate damaged guide thimble upper end portions which effectively addresses potential customer concerns related to improbable, but possible, mishaps which could jeopardize successful fuel reconstitution.